Instead of conventional induction motors (IM motors), permanent-magnet synchronous motors (PM motors) that are highly efficient and are expected to be miniaturized and noise-reduced are spreading. For example, the PM motors have become used as drive motors for electric trains and electric vehicles.
The IM motor generates magnetic flux by an excitation current from a stator, and therefore, has a technical problem of causing a loss when passing the excitation current.
On the other hand, the PM motor is a motor having a rotor provided with a permanent magnet whose flux is used to output torque, and therefore, has no such a problem of the IM motor. However, the PM motor induces, due to the permanent magnet, a voltage depending on the number of revolutions. In the application field of electric trains and vehicles that involves a wide range of rotational speeds, a condition must be secured that a voltage induced at a maximum rotational speed must not break (by overvoltage) an inverter that drives and controls the PM motor. To satisfy the condition, it is necessary to provide the inverter with a sufficiently high withstand voltage, or limit the flux of the permanent magnet of the motor. The former affects a power source, and therefore, the latter is generally chosen. The amount of flux of the PM motor with respect to the amount of flux of the IM motor (in the IM motor, it is the amount of gap flux produced by excitation current) sometimes becomes about 1:3. To generate the same torque, the PM motor of small flux amount must pass a large (torque) current. Namely, in a low-speed zone, the PM motor must pass a larger current than the IM motor, to output the same torque.
Accordingly, the current capacity of an inverter for driving the PM motor must be larger than that for driving the IM motor. The switching frequency of a switching element in the inverter of the PM motor is high, and therefore, the PM motor causes a large loss and heat at low speed because the loss is dependent on a current value.
An electric train, for example, expects to be cooled by a wind created during running, and if a large loss occurs during running at low speed, the inverter must be enlarged to improve the cooling capacity. On the other hand, if an induced voltage is high, field-weakening control must be carried out. In this case, a superposed excitation current deteriorates efficiency.
The PM motor has advantages and disadvantages due to the magnet incorporated therein. As a motor, it has an advantage in reducing a loss and size. On the other hand, for the variable speed control of an electric train or an electric vehicle, the PM motor has operating points at which it shows inferior efficiency compared to the conventional IM motor. For the inverter, increases in the current capacity and loss lead to increase the size of the inverter. Efficiency of the system itself is mainly dependent on the motor, and therefore, employing the PM motor improves the total efficiency of the system. An increase in the size of the inverter, however, is disadvantageous and not preferable for the system.
FIG. 57 is a block diagram showing an example of a permanent-magnet synchronous motor (PM motor) drive system according to a related art. A main circuit consists of a DC power source 3, an inverter 1 to invert DC power into AC power, and a permanent-magnet synchronous motor 4a to be driven by the AC power of the inverter 1. The main circuit is provided with a current detector 2 to detect motor currents and a rotational angle sensor 18 to detect a rotational angle of a rotor of the permanent-magnet synchronous motor 4a. The inverter 1 inverts DC power from the DC power source 3 into AC power, which is supplied to the permanent-magnet synchronous motor 4a. Currents supplied to the permanent magnet synchronous motor 4a are detected by the current detector 2 and are supplied to a voltage command operate unit 210.
Next, control operation of this prior-art system will be explained. An input to the system is a torque command Tm*. This torque command Tm* is generated by a proper means to make the permanent-magnet synchronous motor 4a produce a required torque. According to the input torque command Tm*, a current command operate unit 211 generates a D-axis current command Id* and a Q-axis current command Iq* to determine a D-axis current and a Q-axis current and supplies them to the voltage command operate unit 210. The rotational angle of the rotor of the permanent-magnet synchronous motor 4a detected by the rotational angle sensor 18 is sent to the voltage command operate unit 210. According to the input D-axis current command Id* and Q-axis current command Iq*, the voltage command operate unit 210 calculates and generates D- and Q-axis voltage commands Vd* and Vq* to pass currents in such a way that the D-axis current Id and Q-axis current Iq agree with the current commands. At this time, the voltage command operate unit 210 carries out PI control for a current deviation and finds the D- and Q-axis voltage commands. Thereafter, the voltage command operate unit 210 converts coordinates of the D- and Q-axis voltage commands Vd* and Vq* and provides a PWM circuit 6 with three-phase voltage commands Vu*, Vv*, and Vw*. Although the voltage command operate unit 210 converts the D- and Q-axis voltage commands into the three-phase voltage commands, it is possible to arrange, for example, a coordinate conversion unit to carry out the conversion of the voltage commands. According to the input three-phase voltage commands Vu*, Vv*, and Vw*, the PWM circuit 6 controls ON/OFF of switching elements of the inverter 1.
As shown in FIG. 57, the prior-art PM motor drive system must arrange a load contactor 209 on the AC side of the inverter 1. The permanent-magnet synchronous motor 4a has a permanent magnet, and therefore, induces a voltage (counter electromotive voltage) when the inverter 1 is gated off, as long as the motor rotates due to inertia. If the induced voltage is higher than the DC voltage of the DC power source 3, an overvoltage is applied to the inverter 1 and a braking force is applied to the synchronous motor 4a. 
If the permanent-magnet synchronous motor 4a or the inverter 1 causes a short or an earth fault, the induced voltage will continuously pass a current to cause problems such as the overheating and burning of the permanent-magnet synchronous motor 4a and inverter 1. Accordingly, the above-mentioned drive system opens the load contactor 209 when the inverter 1 is gated off, to prevent the inverter 1 from receiving an induced voltage and the permanent-magnet synchronous motor 4a and inverter 1 from continuously passing a failure current.
A life of the load contactor 209 is greatly dependent on the number of times of open/close operation. When the open/close frequency of the load contactor 209 is high, it suffers from a high failure rate and short life. To improve the reliability of the system, double load contactors 209a and 209b, 209c and 209d, and 209e and 209f may be arranged for respective phases as shown in FIG. 57. This is not a perfect solution and highly increases costs.
Japanese Unexamined Patent Application Publication No. H11-299297 (Patent Document 1) describes a technique of conducting flux-weakening control on a permanent-magnet motor by reducing a flux-weakening current without deteriorating a torque accuracy, to thereby decrease inverter and motor losses and a rated current value of the apparatus. This, however, passes an excitation current to deteriorate efficiency and generate heat. Accordingly, it must have a cooling device, which increases the cost and size of the apparatus.
To solve the problems of the conventional permanent-magnet synchronous motor drive system, Japanese Unexamined Patent Application Publication No. H5-304752 (Patent Document 4) discloses an electric vehicle driving AC motor that employs a combination of a permanent magnet and an excitation coil to change magnetic flux.
The electric vehicle driving AC motor described in the Patent Document 4 efficiently operates the motor and an inverter in each of a low-output operation and a high-output operation. This electric vehicle driving AC motor uses flux of the permanent magnet embedded in a field magnetic pole, and if necessary, flux of the excitation coil, to form field flux. Depending on a motor output, a field flux generation source is switched between only the permanent magnet and both the permanent magnet and excitation coil. At the same time, an excitation current is supplied through a rotational transformer. This electric vehicle driving AC motor can operate in response to a motor output. For example, in response to a low output, it operates only with the permanent magnet, to improve operating efficiency. In addition, it can increase a motor voltage in a low-motor-speed zone, to reduce a current, decrease a copper loss of a motor coil and a generation loss of the inverter, and improve system efficiency. This effect is significant for an electric vehicle that is frequently driven at low and middle speeds, to increase current usage efficiency and extend a driving distance per charge.
In addition, this electric vehicle driving AC motor does not demagnetize the permanent magnet, and therefore, simplifies inverter control, causes no abnormal overvoltage, and protects the system. The rotational transformer may be operated at high frequencies to reduce the size thereof and the size and weight of the system as a whole.
The electric vehicle driving AC motor stipulated in the Patent Document 4, however, must always pass an excitation current when generating flux by the excitation coil. Passing an excitation current causes a problem of deteriorating efficiency, and the permanent magnet embedded in the field magnetic pole causes a problem of inducing a voltage.
In connection with this, there is a variable-flux motor drive system capable of varying the flux of a magnet with a current from an inverter. This system changes the amount of flux of a permanent magnet according to operating conditions, and therefore, is expected to improve efficiency more than the conventional fixed magnet PM motor drive system. When the magnet is not needed, the flux amount can be reduced to minimize an induced voltage.
For the variable-flux motor drive system for driving a variable-flux motor whose flux is variably controllable with a magnetizing current from an inverter, it is important when and how the magnetization process to change flux is carried out. Transient torque will occur depending on a torque accuracy or the magnetization process. The magnetization process must be carried out at the timing to maximize efficiency and expand a speed range.
The BH characteristic (magnetization-flux density characteristic) of a variable magnet shows a steep response to a magnetizing current from an inverter, to easily fluctuate flux depending on a way of magnetization. The flux fluctuation deteriorates a torque repeatability and the quality of the drive system.
To magnetize the variable magnet, the inverter must pass a large current. In this case, a stator will saturate. Compared with demagnetizing the variable magnet, magnetizing the variable magnet needs a larger current to be passed. The need of a large current increases the current capacity of each switching element of the inverter, and also, each switching element of the inverter must have a higher withstand voltage. Namely, only for the magnetization process, each switching element must have a large capacity to increase the cost thereof. In addition, the large current produces instantaneous heat, and therefore, the thermal capacity of the inverter must be increased so that the inverter may resist against the short-time heat.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H11-299297    Patent Document 2: U.S. Pat. No. 6,800,977    Patent Document 3: U.S. Pat. No. 5,977,679    Patent Document 4: Japanese Unexamined Patent Application Publication No. H5-304752